1. Field of The Invention
The present invention is directed to a method of mitigating gain peaking in optically amplified systems having application in long-haul transmission systems, networks and other optical communication systems. Specifically, the present invention is directed to maintaining adequate signal-to-noise ratio (SNR) and gain over a range of wavelengths, mainly those in which wavelength-multiplexed (WDM) optically amplified channels will be present. The method of the present invention achieves the desired results by using a chain of fiber amplifiers and pumping them at a predetermined wavelength to produce gain over a specified wavelength range covering the range of likely channels for WDM systems.
2. Prior Art Statement
There is considerable interest in using dense wavelength division multiplexing (DWDM) and optical amplification (Nagel et al.) in optical networks, both for enhancing the capacity of existing long-haul optical networks or for achieving high capacity in future local area networks. One area of great concern is ensuring that, in systems containing concatenated chains of optical amplifiers, an adequate optical signal-to-noise ratio (SNR) is maintained over the range of wavelengths that the WDM channels will occupy. The problems of gain and/or SNR equalization have received considerable attention. K. Inoue, T. Kominato and H. Toba, "Tunable Gain Equalization using a Mach Zehnder Optical Filter in Multistage Fiber Amplifiers", IEEE Photonics Technology Letters, Vol. 3, 1991, pp. 718-720, have approached the problem by using Fabry-Perot and Mach-Zehnder filters. A more useful and simpler technique of adjusting the power in each channel at the transmitters using information conveyed via telemetry was utilized by A. R. Chraplyvy, J. A. Nagel and R. W. Tkach, "Equalization in Amplified WDM Lightwave Transmission Systems", IEEE Photonics Technology Letters, Vol. 4, 1992, pp 920-922. While this technique works, it would clearly be preferable if it was unnecessary. It has recently been shown by E. Goldstein, V. daSilva, L. Eskilden, M. Andrejco and Y. Silberberg, "Inhomogeneously Broadened Fiber-Amplifier Cascade for Wavelength-Multiplexed Systems", IEEE Photonics Technology Letters, Vol. 5, 1993, pp. 543-545, that a chain of amplifiers will produce a flatter gain profile if they are inhomogeneously broadened. However, this requires cooling the erbium-doped fibers to low temperatures. These studies have all dealt with erbium-doped fiber amplifiers pumped at a wavelength of 1480 nm. Therefore, the prior art does not disclose a method for mitigating gain peaking by providing a chain of fiber amplifiers and pumping the amplifiers at a predetermined wavelength, preferably at a wavelength less than 1400 nm, to achieve gain over a specified wavelength range.